The invention relates to air powered hydraulic lifting systems and more particularly to controls for effecting actuation of such systems.
In hydraulic lift systems it is advantageous in some applications to use an air pressure driven motor to drive a hydraulic fluid pump which selectively supplies hydraulic fluid pressure to the hydraulic cylinder of the hydraulic lift system. An example of a prior art arrangement is illustrated in U.S. Pat. No. 4,251,055. Attention is also directed to U.S. Pat. No. 4,889,472.
In prior art arrangements of such hydraulic lift systems a manually operated control valve is provided to control the flow of hydraulic fluid from the pump to the cylinder. A pressure relief valve is connected between the pump and the fluid cylinder to exhaust fluid to the tank when fluid pressure reaches a predetermined pressure. If the cylinder reaches the end of its stroke and the hydraulic pump continues to operate, hydraulic fluid is exhausted to the tank through the pressure relief valve to the tank. This can result in damaging heat generation in the hydraulic system and destruction or deterioration of components of the system such as seals and other non-metallic components.
The invention provides an improved fluid motor powered fluid lift system having a valve arrangement for reducing or interrupting the fluid supplied to the fluid motor when the pressure in the lift system exceeds a predetermined pressure.
One of the features of the construction of the invention is that a valve arrangement is provided to shut-off or throttle back the fluid supplied to the fluid motor when the fluid pressure reaches a predetermined level. When the pressure of the lift system exceeds the predetermined level, pilot pressure is supplied to an automatic shut-off valve, and this in turn shuts off fluid flow to the fluid motor thereby preventing the pump from continuing to supply pressure that would otherwise be bypassed by relief valves to exhaust back to the reservoir.